Glock 26 gen 4 problems

Hello, I've been shooting glocks for about 6 years now. I've owned the 27 and the 21. I now own the gen 4 17 and 26.

My 26 has around 500 rounds down the pipe. It has given me no problems until yesterday. We were shooting wwb ammo with the factory 10 round mag. Near the end of the session the slide started locking back halfway through the mag. It did this to me and my friends. It kept doing this and towards the end it was locking back every other round.

I know that an improper grip can cause this but since I've never had this problem before I think it's mechanical. Does anyone know what could be causing this? Thanks for any help.